Multiple-harmonic generation in rare gases at high laser intensity

We present experimental measurements of vacuum ultraviolet light emission processes in a 15-Torr rare-gas medium exposed to a strong 1064-nm laser field. Apart from a small number of lines which correspond to discrete transitions, and a broad continuum emission, we essentially observe the odd harmonics of the laser field, up to very high order. At a 3×${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{-}2}$ laser intensity, the highest-order harmonics observed are the 33rd in Ar, the 29th in Kr, and the 21st in Xe. The harmonic distribution presents three regions: a steep decrease for the first harmonics, a plateau, and a sudden cutoff at high order. We study the variation of the conversion efficiency for some of the harmonics as a function of the incident laser intensity and the atomic density. Finally, we analyze the harmonic-generation results as thoroughly as possible, in particular, by characterizing the phase-matching conditions in our experiment.